The ability of tendons to heal after injury is limited. A damaged tendon can cause significant pain and disability, and can ultimately lead to instability and deterioration of the surrounding joint. The tendons of shoulder joint (i.e., the rotator cuff) are particularly susceptible to injury, with tears often occurring near the tendon to bone interface after significant chronic tendon deterioration. No current surgical repair technique has been entirely successful in repairing these injuries. This may largely be due to the poor quality of the injured tissue at the time of surgical repair. A tissue-engineering construct combining a bioresorbable material with a cellular component could ultimately provide a solution for this tissue injury. The goal of this study is to determine the relationships between tissue structure, composition, and viscoelastic properties in tendon fibroblast populated collagen gels. With this knowledge, a tissue-engineered tendon equivalent can be designed in vitro to meet the functional demands of native tissue. Special emphasis will be placed on recreating the viscoelastic properties of the tissue, which are necessary for proper function. The aims of this study are: (1) determine the relationship between structure (i.e., collagen alignment) and viscoelastic properties, (2) determine the relationship between composition (i.e., extracellular matrix components) and viscoelastic properties, and (3) determine the relationship between external loading (i.e., static or cyclic loading) and viscoelastic properties, in tendon fibroblast populated collagen gels. This approach will use viscoelastic properties as the functional goal, and vary the structure and composition to achieve this goal. It is expected that a combination of structural and compositional design will be necessary to achieve the viscoelastic properties of normal tendon.